The present invention relates generally to the field of optical media, and more specifically to the area of optical media which employ two or more information storage layers.
There is a seemingly never-ending demand in the field of data storage for media having increased storage capacity and performance. In the field of pre-recorded optical discs, such as compact discs and video discs, increased storage capacity is usually achieved by increasing the storage density per unit area of the disc. However, the maximum data storage density achievable in an optical recording system is limited by the smallest feature that the optical system can resolve. For conventional far-field imaging systems, the smallest resolvable feature size is limited by diffraction effects to approximately the wavelength of the available light source, usually a solid state laser diode. Thus, one method of increasing disc storage capacity is to decrease the wavelength of the laser diode. However, while the wavelengths available from laser diodes have been steadily decreasing, the decreases have not been dramatic due to limitations in solid state technology and materials.
A number of other techniques for increasing storage capacity of optical recording systems have been proposed. These include: (1) higher efficiency data coding schemes, e.g., pulse-width modulation; (2) optical and/or magnetic super-resolution; (3) zoned recording at constant angular velocity; (4) advanced data channel detection methods, such as partial response/maximum likelihood detection, and (5) recording on both the grooves and land areas of the disc.
While the preceding methods for increasing storage capacity all rely upon increasing the storage density per unit area of the disc, an alternative method for increasing the capacity of an optical disc is to employ additional storage layers on the disc which can be independently recorded or reproduced. Thus, the approach in this case is to increase the addressable area of the disc. This approach is attractive because it has the potential to substantially increase media storage capacity with only a modest increase in media and recording system complexity.
If multiple storage layers, e.g., 2, are to be read and/or written by optical beam(s) provided on one side of the disc, then one of the storage layers of the disc must be reflective enough so that it may be read and/or written by the optical beam(s), but transparent enough so that the beam(s) may penetrate the first storage layer and pass on to a second storage layer. However, such a disc has proved to be difficult to construct.
Accordingly, the present invention is directed to an optical storage medium having a partially reflective layer and a highly reflective layer, whereby data/servo information/format information may be stored on two different layers of the medium. In one embodiment, the medium includes an ordered stack of a transparent substrate, a partially reflective layer, a transparent spacer layer, and a highly reflective layer. The substrate has a pattern of pits in one of its major surfaces. The partially reflective layer may be adjacent the pit pattern side of the substrate. The partially reflective layer comprises amorphous selenium (a-Se). The partially reflective layer may also consist essentially of a-Se, which may be deposited in a variety of manners, including radio frequency (RF) sputtering, such as RF magnetron sputtering.
Another embodiment of the present invention includes a dual layer pre-recorded optical storage disc, comprising, in order, a transparent substrate, a partially reflective layer, a transparent spacer layer, and a highly reflective layer. A first data pit pattern is provided on one side of the disc. The partially reflective layer is adjacent the first pit pattern and has an index of refraction having a real component, n, and an imaginary component, K, wherein n greater than 2.6 and K less than 0.035 when measured at 650 nm. A second data pit pattern is provided between the transparent spacer layer and the highly reflective layer. The imaginary component, K, of the refractive index is preferably less than 0.01, and more preferably less than 0.003. The real component, n, of the refractive index is preferably greater than 2.8, and more preferably greater than 3.0. The thickness of the partially reflective layer is preferably the same as those discussed above for the partially reflective layer comprising a-Se.
The embodiments of the inventive optical storage media described above each have two aspects. In one aspect, the medium is designed to carry two or more layers of data/servo/format information which may be read by a drive capable of focusing on each of the two or more information layers. In this aspect, an optical storage system for use with the media would include the media as described above, a focused laser beam positioned to enter the medium through the substrate, means for adjusting the focal position of the laser beam, whereby the beam may be focused on either the partially reflective layer or the highly reflective layer, and a photodetector to detect the reflected laser beam exiting the medium. In this aspect, the preferred thickness for the partially reflecting layer is within the range from about 38 to 69 nm, more preferably 42 to 64 nm.
In the second aspect of the present invention, the optical storage medium is a disc which is designed for use with two different drives. The entire disc has a nominal thickness of 1.2 mm so that the pit pattern in the highly reflective layer may be read by a CD-ROM (compact disc-read only memory) drive having a 780 nm laser. The disc also has a substrate having a nominal thickness of about 0.6 mm, so that the pit pattern in the partially reflective layer may be read by a DVD-ROM (digital versatile disc) drive having a 650 nm laser. This would allow a pre-recorded disc seller to sell one disc that could be read by a consumer owning either a CD-ROM or DVD-ROM drive. (Of course, the CD-ROM version of the information would be expected to be less elaborate since the CD-ROM format does not allow for the same storage capacity as the DVD-ROM format.)
In this second aspect, the partially reflective layer preferably has a thickness in the range from about 130 to 160 nm, more preferably 140 to 160 nm. The reflectivity, R1, of the highly reflective layer is preferably greater than 0.7 for 780 nm light, and the reflectivity, R2, of the partially reflective layer is preferably between 0.2 and 0.4 for 650 nm light.